DIRECT EVIDENCE FOR CONDENSATION IN THE EARLY SOLAR SYSTEM AND IMPLICATIONS FOR NEBULAR COOLING RATES

We have identified in an acid resistant residue of the carbonaceous chondrite Murchison a large number (458) of highly refractory metal nuggets (RMNs) that once were most likely hosted by Ca, Al-rich inclusions (CAIs). While osmium isotopic ratios of two randomly selected particles rule out a presolar origin, the bulk chemistry of 88 particles with sizes in the submicron range determined by energy dispersive X-ray (EDX) spectroscopy shows striking agreement with predictions of single-phase equilibrium condensation calculations. Both chemical composition and morphology strongly favor a condensation origin. Particularly important is the presence of structurally incompatible elements in particles with a single-crystal structure, which also suggests the absence of secondary alteration. The metal particles represent the most pristine early solar system material found so far and allow estimation of the cooling rate of the gaseous environment from which the first solids formed by condensation. The resulting value of 0.5 K yr(-1) is at least 4 orders of magnitude lower than the cooling rate of molten CAIs. It is thus possible, for the first time, to see through the complex structure of most CAIs and infer the thermal history of the gaseous reservoir from which their components formed by condensation.